Pneumatic conveying of various materials or products by a pneumatic fluid through transport lines or transport pipes of a pneumatic conveying or transport system is widely used. Generally, the pneumatic fluid is a gas and in most cases the fluid is air.
Pneumatic conveying systems for the transport of fibrous material, for example, can be categorized into various modes of operation. A first mode of operation comprises continuous feeding, or at least batch type feeding, wherein it is an object to maintain a continuous flow of material through a transport line, at least for the amount of the batch of material to be transported. A second mode of operation is a demand driven start-stop process, by which the fibrous material is conveyed through the transport line in one of suspension flow, by which the fibres are conveyed apart from each other, by separately spaced dunes of material or as a substantially continuous layer of material comprising separately spaced dunes, also called dune and layer flow.
Both modes may suffer from plugging, in that the material or product in the transport line clogs, such that transport through the transport line may come to a complete standstill, or at least in that the conveying speed of the material is significantly reduced. Plugging can be avoided by decreasing the amount of material compared to the amount of pneumatic fluid in the transport line. However, if the ratio of the amount of material and the amount of air becomes too low, the effective conveying capacity of the conveying system decreases.
The business case in modern pneumatic conveying systems is in general to operate the system at an optimum with respect to the consumed or added fluid, such as air, and the amount of material transported in a given time period, while avoiding plugging and damaging of the material to be transported, among others as a result of a too high transport speed or transport velocity of the material in the transport line. Reducing damaging of the transport line and other appliances of the conveying system, such as valves, speed and pressure sensors, couplings and the like should, of course, also be taken into account when setting control parameters of the conveying system.
Operation of a pneumatic conveying system in start-stop mode, such as with the conveying of fibrous material like cut tobacco in suspension flow in a device for feeding cut tobacco to a tobacco processing device, such as a cigarette maker, in particular introduces a huge product degradation risk as each tobacco fibre is separately exposed to mechanical stresses during transport. On the other hand, the overall degradation risk of the tobacco fibres in dune or dune and layer flow conveying is reduced, as the fibres inside a dune or layer as such encounter less degradation.
Solutions provided to tobacco factories today are generally based on constant pressure conveying, which leads to uncontrolled velocity peaks due to variation in product loading.